LONDON (Reuters) - HIV epidemics are becoming more concentrated in marginalized groups such as sex workers, drug users and gay men, and could defy global attempts to combat AIDS if attitudes do not change, a U.N. expert said.

Michel Kazatchkine, U.N. Special Envoy for HIV/AIDS in Eastern Europe, says he would like to be able to celebrate without reservation global progress made in the past decade, but stubborn infection rates and alarming growth of outbreaks in hard-to-reach populations make that difficult.

The risk, he says, is that as the world turns the tide of the global AIDS epidemic, the virus will return to being a disease that plagues only certain groups and the political will to overcome it may fade.

"If we do not address the roots of the problem, if we do not address stigma, discrimination and inappropriate legislation, if we don't look at these people from a public health perspective, rather than from a delinquent, criminal perspective as we do now, then the trend will only go on," he said in an interview.

"Then the AIDS epidemic will become more and more a sum of these concentrated epidemics."

EXTRAORDINARY PROGRESS

Some 35.3 million people worldwide are infected with the human immunodeficiency virus (HIV) that causes AIDS, but the rising number of patients reflects great strides in recent years in developing sophisticated HIV tests and combination AIDS drugs and getting them to many of those who need them to stay alive.

The annual AIDS death toll is falling, dropping to 1.6 million people in 2012, down from a peak of 2.3 million in 2005, and there are also steadily declining rates of new HIV infections: a third fewer in 2013 than in 2011.

The progress has generated much hope about the possible end of AIDS, or a potential world without HIV, or the chance of an AIDS-free generation, in our lifetimes.

Kazatchkine, who was due to speak at a City Health Conference in the Scottish city of Glasgow on Monday, refers to both the progress and the hope as "extraordinary."

"I'm really concerned about the future of the AIDS epidemic, especially at a time when we are perhaps a little too optimistic because of the huge progress we are making from a technological and scientific perspective," he said.

"As we celebrate the extraordinary progress, we should also be conscious that we will not stop HIV and AIDS by just having more sophisticated drugs and only focusing on the generalized epidemic and not focusing enough on the complexities of the concentrated epidemics."

The worrisome groups are fairly clearly defined: injecting drug users, who can pass the AIDS virus to each other by sharing needles and syringes, prostitutes and sex workers, who are often criminalized and have little access to health services, and gay and bisexual men - the population in which the HIV epidemic started.

A TALE OF TWO WOMEN

To illustrate how little has changed in the battle against HIV among drugs users - particularly in regions such as Eastern Europe and central Asia - Kazatchkine tells of two women.

The first is Andr�e, a drug user he met in Paris in 1986 who had no hope of effective HIV treatment, since there was none yet developed, and who ultimately died a lonely death. The second was Larissa from Yekaterinburg in Russia, a drug addict repeatedly arrested and locked up, deprived of medications for years and at one time sentenced to five years in a labor camp.

"These stories are remarkably similar," he said. "But Larissa's is not from 1986, it's from this year. Some 25 years passed between my meeting these two women, but their predicament was depressingly, tragically, the same."

Among gay men, Kazatchkine said, the situation is little better. In poor and middle-income countries, men who have sex with men (MSM) and female sex workers are 19 and 13 times more likely to have HIV, respectively, than the rest of the population.

Even in wealthy regions like Western Europe and North America, HIV rates among MSM stubbornly refuse to shift.

HIV AMONG GAY MEN

"In MSM populations, there is no sign it has decreased," he said. "It has either been a stable number of new infections every year for 10 years, or it is an increasing trend. And this, in Western Europe at least, is in the context of basically free and easy access to therapy and services."

Elsewhere, in China, for example, gay men alone account for more than 33 percent of new HIV infections, and projections indicate that gay men may account for half or more of all new infections in Asia by 2020.

Kazatchkine admits that he is as frustrated and worried now, faced with these smaller but relentless HIV epidemics, as he was more than a decade ago when the vast generalized HIV and AIDS outbreak in Africa looked too overwhelming to begin to tackle.

"We are a bit in disarray. We don't know quite what it is that we should do. Here we are, we have all the technology, we have extraordinary scientific progress, and we just cannot translate that into making a difference in these populations."

Yet if nothing changes, the AIDS virus may never be beaten.

Kazatchkine called for a "shift in the collective mindset" to put equity and human rights at the center of the battle against HIV in these groups: "If we do not deliver the right response, we will fail to deliver an end to AIDS," he said.